1. Field of the Invention
The present invention relates to an optical deflection device for deflecting a light beam, such as a laser beam, which is used as an element of an optical system for recording digital data on and reading the recorded digital data out of an optical disk or optical-magnetic disk.
2. Description of the Related Art
As a representative optical deflection device, a galvano-mirror device is well known, as disclosed in, for example, Examined Japanese Utility Model Publication No. 7-28585 corresponding to U.S. Pat. No 5,249,173. This galvano-mirror device or optical deflection device comprises a yoke member having two opposed projections protruding from the ends thereof, and a movable member supported by a pair of leaf springs securely attached to the opposed projections of the yoke member. The movable member includes an electromagnetic coil, which operates in conjunction with permanent magnets, supported by the yoke member, such that the electromagnetic coil is interposed between the permanent magnets. The movable member further includes a mirror element securely attached thereto. The pair of leaf springs is arranged so as to define a rotational axis passing through the center of gravity of the movable member.
When an electrical current flows in the electromagnetic coil, the electromagnetic coil, and therefore the movable member, is subjected to a rotational force in accordance with Fleming's Rule. The movable member is rotated around the rotational axis in a rotational direction defined by a direction of flow of the electrical current. A degree of the rotation of the movable member depends upon an amount of electrical current flowing through the electromagnetic coil. Thus, it is possible to deflect a light beam, made incident on the mirror of the movable member, in a desired direction by controlling the amount of electrical current flowing through the electromagnetic coil.
Nevertheless, in the optical deflection device, as disclosed in U.S. Pat. No. 5,249,173, the leaf springs are susceptible to structure-coupled-oscillations and thrust-coupled-oscillations, resulting in the optical deflection device being subjected to harmful influences, such as dynamic crosstalk. The induced oscillations occur due to the high compliance exhibited by each of the leaf springs in the direction perpendicular to the main surface of the leaf spring.
Note, the structure-coupled-oscillations are defined as coupled-oscillations produced in the leaf springs due to outside forces exerted upon the optical deflection device, and the thrust-coupled-oscillations are defined as coupled-oscillations produced in the leaf springs due to rotational forces of the movable member.
Examined Japanese Patent Publication No. 3-49409 discloses another type of optical deflection device which comprises a base member, and a movable member supported, at opposed sides thereof, by two sets of crosswise leaf springs securely attached to the base member. The movable member includes two electromagnetic coils provided at the other opposed sides thereof, respectively, and the electromagnetic coil operates in conjunction with permanent magnets provided beside the other opposed sides of the movable member. The movable member further includes a mirror element securely attached to the movable member. In this optical deflection device, the movable member is also rotated in accordance with Fleming's Rule around a rotational axis defined by the two sets of crosswise leaf springs.
Although the two sets of crosswise leaf springs are less susceptible to the structure-coupled-oscillations and the thrust-coupled-oscillations than the leaf springs disclosed in U.S. Pat. No. 5,249,173, due to the crosswise arrangement of the leaf springs, the crosswise leaf springs are still prone to the structure-coupled-oscillations and the thrust-coupled-oscillations. In particular, the crosswise leaf springs are securely attached to the base member and the movable member, and, due to these locations of attachment being spaced radially away from the rotational axis of the movable member, the oscillations occur.
Also, in assembling the optical deflection device, it is difficult to precisely align the intersection point of the crosswise leaf springs with the rotational axis of the movable member, because of the inherent constructional arrangement of the optical deflection device. Further, the movable member must have two respective sets of block elements, to which the two sets of crosswise leaf springs are securely attached. Of course, the moment of inertia of the movable member is increased due to the addition of the block elements thereto, resulting in a deterioration in the responsiveness of the rotational movement of the movable member, and therefore the mirror.